Fluoropolymers are applied to a wide number of substrates in order to confer release, chemical and heat resistance, corrosion protection, cleanability, low flammability, and weatherability. Coatings of polytetrafluoroethylene (PTFE) homopolymers and modified PTFE provide the highest heat stability among the fluoropolymers, but unlike tetrafluoroethylene (TFE) copolymers, cannot be melt processed to form films and coatings. Therefore other processes have been developed for applying coatings of PTFE homopolymers and modified PTFE. One such process is dispersion coating which applies the fluoropolymer in dispersion form. Dispersion coating processes typically employ such fluoropolymer dispersions in a more concentrated form than the as-polymerized dispersion. These concentrated dispersions contain a significant quantity of surfactant, e.g. 6-8 weight percent. Such dispersion coating processes include the steps of applying concentrated dispersion to a substrate by common techniques such as spraying, roller or curtain coating; drying the substrate to remove volatile components; and baking the substrate. When baking temperatures are high enough, the primary dispersion particles fuse and become a coherent mass. Baking at high temperatures to fuse the particles is often referred to as sintering. For a number of dispersion coating applications such as curtain coating or seriography, a fraction of the concentrated dispersion coating stream is deposited on the substrate requiring the remainder of the stream to be recycled. The recycled fraction needs to be able to withstand the subsequent multiple pumping and mixing operations necessary for a continuous process. A dispersion suitable for such processing should not easily coagulate when subjected to shearing forces. The resistance of the dispersion to premature coagulation can be measured by a parameter known as gel time and is an indication of the shear stability of the dispersion.
In commercial dispersion coating processes, polymers with moderate molecular weights have typically been used, i.e., having a melt creep viscosity of approximately 1.2×1010 Pa·s. For some applications such as PTFE coated fiberglass cloth, it is desired to use higher molecular weight polymer than has typically been used commercially to obtain improvements in flex life and increased lifetimes in industrial and/or architectural applications. However, if higher molecular weight polymers are used in a commercial process which subjects the polymer to significant shear forces, shear stability is adversely affected.
Fluoropolymers are known which have particles of a high molecular weight core of fibrillating fluoropolymer and a low molecular weight core of nonfibrillating polymer. Because of the very low molecular weight, non-fibrillating shell on these polymers, resins powders of these polymers are not fibrillating polymers. For example, U.S. Pat. No. 5,324,785 (Noda et al.) discloses a dispersion of core-shell fluoropolymer particles in which the core is high molecular weight fibrillating PTFE and the shell is nonfibrillating PTFE. The shell of this polymer is disclosed to have a molecular weight of 10,000 to 800,000 and a melt viscosity of 102 to 106 poises. The objects of the invention included providing fine particles or powder and a sintered PTFE powder which have a high molecular weight and good blending or dispersion properties in a resin, an elastomer, a paint, etc. European Patent Publication EP 0 758 010 A1 discloses a similar composition for use as a anti-drip agent for imparting a flammable thermoplastic resin with anti-drip property. U.S. Pat. No. 5,707,763 (Shimizu et al.) discloses a similar polymer for use as a binder for battery electrodes with a fibrillating PTFE core and a shell of any of variety of nonfibrillating fluorine-containing polymers or copolymers.
While advantageous for use in blending powders of the above-described resins with other materials, the presence of the very low molecular weight, non-fibrillating polymer in the shell creates problems in dispersion coating applications. A high molecular weight PTFE dispersion is needed which confers the benefits of high molecular weight PTFE while providing good shear stability necessary for continuous commercial dispersion coating processes.